Question: An equilateral triangle $PQR$ is inscribed in the ellipse $\frac{x^2}{a^2} + \frac{y^2}{b^2} = 1,$ so that $Q$ is at $(0,b),$ and $\overline{PR}$ is parallel to the $x$-axis, as shown below.  Also, foci $F_1$ and $F_2$ lie on sides $\overline{QR}$ and $\overline{PQ},$ respectively.  Find $\frac{PQ}{F_1 F_2}.$

[asy]
unitsize(0.4 cm);

pair A, B, C;
pair[] F;
real a, b, c, s;

a = 5;
b = sqrt(3)/2*5;
c = 5/2;
s = 8;

A = (-s/2,-sqrt(3)/2*(s - 5));
B = (0,b);
C = (s/2,-sqrt(3)/2*(s - 5));
F[1] = (c,0);
F[2] = (-c,0);

draw(yscale(b)*xscale(a)*Circle((0,0),1));
draw(A--B--C--cycle);

label("$P$", A, SW);
label("$Q$", B, N);
label("$R$", C, SE);
dot("$F_1$", F[1], NE);
dot("$F_2$", F[2], NW);
[/asy]
Explanation: Without loss of generality, assume that $F_1 F_2 = 2,$ so $c = 1.$  Since triangle $QF_1 F_2$ is equilateral, $b = \sqrt{3}$ and $a = 2.$

[asy]
unitsize(0.4 cm);

pair A, B, C, M;
pair[] F;
real a, b, c, s;

a = 5;
b = sqrt(3)/2*5;
c = 5/2;
s = 8;

A = (-s/2,-sqrt(3)/2*(s - 5));
B = (0,b);
C = (s/2,-sqrt(3)/2*(s - 5));
F[1] = (c,0);
F[2] = (-c,0);
M = (A + C)/2;

draw(yscale(b)*xscale(a)*Circle((0,0),1));
draw(A--B--C--cycle);
draw((-a,0)--(a,0));
draw((0,-b)--(0,b));

label("$P$", A, SW);
label("$Q$", B, N);
label("$R$", C, SE);
dot("$F_1$", F[1], NE);
dot("$F_2$", F[2], NW);
label("$c$", (c/2,0), S);
label("$a$", (c/2,b/2), NE);
label("$b$", (0,b/2), W);
label("$M$", M, SW);
[/asy]

Let $s$ be the side length of equilateral triangle $PQR,$ and let $M$ be the midpoint of $\overline{PR}.$  Then $RM = \frac{s}{2}.$  Also, $RF_1 = QR - QF_1 = s - 2,$ so the distance from $R$ to the $x$-axis is $\frac{\sqrt{3}}{2} (s - 2).$

Hence, $R = \left( \frac{s}{2}, -\frac{\sqrt{3}}{2} (s - 2) \right).$  Substituting these coordinates into the equation of the ellipse, we get
\[\frac{(\frac{s}{2})^2}{4} + \frac{(-\frac{\sqrt{3}}{2} (s - 2))^2}{3} = 1.\]This simplifies to $5s^2 = 16s,$ so $s = \frac{16}{5}.$  Therefore,
\[\frac{PQ}{F_1 F_2} = \frac{16/5}{2} = \boxed{\frac{8}{5}}.\]